The state of the art of portable television devices has progressed from thirty to fifty pound black and white units of the 1950's having twelve inch screens through modern color units of similar weight and having nineteen inch screens. In addition to these "portables" of relatively gigantic proportions which are more properly termed "transportable" or "mobile", there have been easily carried units with screens ranging down to about one and a half inches for black and white screens and about three inches for color screens. The reductions in size for portable televisions and the increase in picture quality has been the result of the trend away from tube type circuitry to transistor circuitry and more recently to integrated circuitry, in addition to decreases in the sizes of picture tubes.
The current trend in portable televisions is toward almost pocket sized units which employ integrated circuitry and so-called flat panel displays. There are several types of flat panel displays available and under development including, for television purposes, flattened cathode ray tubes (CRT's) and various types of monochromatic and color liquid crystal displays (LCD's).
In one type of flattened CRT, the phosphor screen is positioned at an angle to the electron beam and is viewed from the surface struck by the beam. Less power is required for such CRT's than for conventional CRT's since it is not necessary for the phosphors to be excited to a level required for the glow to penetrate to the other side of the phosphor layer. In color LCD's, the image is formed by an array of transparent picture elements or pixels, each pixel comprising a triad of a red, a green, and a blue element. Early LCD's had only an ON state and an OFF state; however, newer displays are capable of a range of densities from off to on, a gray scale in photographic terms. Since LCD's do not generate their own light, backlighting is required for viewing the displayed image.
There are trade-offs in the advantages of one flat screen technology over the other. The LCD's require much less power than the flattened CRT's since it is not necessary to generate an electron beam and since LCD's are inherently low power devices. However, while the scan generating circuitry of flattened CRT's is nearly conventional requiring a slight modification for the orientation of the screen, the image generation for LCD's is much more complex, requiring sequential addressing of the pixels and a required voltage level. Currently, the image quality of flattened CRT's is generally superior to that of LCD's.
Regardless of the type of image display device, current portable televisions do not differ in overall concept from conventional television sets: namely, a television receiver with one screen and one speaker. While there have been a few attempts to include stereophonic sound on video broadcasts, the practice is not widespread. And while videocassette recorders (VCR's) may be played through most televisions, it is necessary to modulate a carrier signal which can be tuned by the television receiver and then down convert and detect the program signal. These extra signal processing stages introduce noise resulting in the degradation of the quality of the recorded video signal.
The art of recording television signals has similarly progressed from a two inch magnetic tape format in broadcasting studios only to the current half inch and eight millimeter videocassette recorder formats available for home use. One of the latest developments in video recording is the emergence of the so-called "camcorder" which is a video camera and recording system in one unit. In contrast to earlier systems in which the camera fed a video signal to a separately packaged recorder unit which contained a full sized videocassette, the camcorders employ videocassettes of reduced size such that the entire camera, recorder, and cassette package are only slightly larger than previous videocassette cartridges.
In addition to the magnetic tape recording of television signals, there have been developments in the recording of such signals on nonmagnetic discs in much the same manner that musical performances are recorded on phonograph records. In one video disc format, the signal is recorded as a spiral groove on the surface and is read by a stylus as with phonograph records. In another video disc format, the signal is recorded as a spiral track of reflective depressions or pits which are read optically by a laser beam. One major advantage of an optically read disc is that there is no wear of the medium as it is read as occurs with magnetic tapes and groove type discs. The majority of current video discs have a diameter of twelve inches. There are currently several commercially available video discs having smaller diameters including approximately ten inches and eight and one half inches.
In the field of sound recording, a recent development is the compact digital audio disc. Compact discs have a diameter of less than five inches and are written and read in much the same manner as optical video discs. However, the sound signal is encoded digitally which greatly reduces the noise content and allows much greater dynamic range than can be accomplished in magnetic recording and in groove type phonograph records. Because of the small size of the compact disc medium and their playerback devices and because of developments in the tracking mechanisms therefor, compact disc players are now available in portable models.